The present invention relates generally to metal components and more particularly relates to metal components suitable for use in or near nuclear reactors.
The properties of metal components in a nuclear reactor are affected by radiation exposure. For safety reasons, the extent of irradiation-induced change in reactor parts can be a significant factor in reactor design.
One component of a nuclear reactor system in which consideration of radiation-induced changes is particularly important is in the fuel assembly. In a conventional reactor, the fuel is contained in rods which are grouped together and held in place by fuel assembly grids. The grids are structured to provide an individual channel for each rod. Coolant is circulated through the channels along the outer surface of the fuel rods. Adequate flow of coolant is needed in order to keep the fuel rods from overheating.
Each rod in a fuel assembly grid is held in place within a particular channel by springs, usually cantilever or arched springs. The springs are specially designed to result in minimal disruption to the flow of coolant around the rods, while supporting the rods strongly enough to prevent vibration or longitudinal displacement due to flow forces. Cantilever springs frequently are preferred over arched springs because grids containing cantilever springs have less blockage of coolant flow, and can be made shorter, than grids containing arched springs. Both of these factors contribute to the desirable result of a relatively low pressure drop across grids having cantilever springs. The springs often are made of zircaloy, a zirconium-tin alloy.
Cantilever springs made from zircaloy have a disadvantage, however, in that they tend to relax after only a short period of irradiation. Frequently, the load on the springs decreases to zero during one operational cycle of the fuel assembly. While the use of a high initial load may result in a somewhat longer retention period for a positive spring force, the degree of improvement is not substantial. In any event, the use of springs having a high initial load can be disadvantageous because the fuel rods must be inserted very carefully in order to avoid scoring the cladding of the rods.